Injecting remains a primary source of HIV-related harms globally, and people who inject drugs (PWID) have been shown to play a key role in the initiation of other individuals into injection drug use. However, no interventions to prevent injecting initiation by focusing on the role of PWID have to date demonstrated effectiveness, likely because they focus on individual-level behavior change rather than on modifying the social and structural factors that influence PWID to initiate others. Indeed, data from multiple urban centers in North America suggest that ongoing exposure to injecting increases the risk that populations at risk such as street youth initiate injecting, and that the majority of non-injectors are initiated into injecting by PWID. Reducing the exposure of street youth to injecting is therefore likely to reduce their risk of initiation into injection. We hypothesize that intervenions that reduce the frequency of public injecting may therefore have a secondary preventive impact on injecting initiation by limiting the exposure of non-injectors to injecting. This multi-country, mixed methods project will therefore investigate whether a range of interventions and factors shaping the risk environment for injecting risk among PWID (opioid substitution therapy, supervised injection facilities, stable housing, incarceration environments, public injecting, and gender) may also influence the risk that they initiate others into injecting. This study will seek o: 1) determine the prevalence and characteristics of PWID participation in injecting initiation across multiple study sites; 2) assess factors potentially influencing the risk that PWID facilitat injecting initiation; 3) investigate individual pathways and roles that increase the risk of PWID facilitating injecting initiation; and 4) test the impact of an existing structural or biomedical intervention on the risk that PWID will facilitate injecting initiation. To accomplish this, we wil conduct a prospective, multi-site study of PWID (n = 3,050) in North America and France employing quantitative and qualitative data from four separate cohort studies of PWID (San Diego, STAHR II; Tijuana, El Cuete IV; Vancouver, VIDUS II; Paris, Marseille, and Bordeaux, COSINUS). This project is the first study investigating whether biomedical and structural interventions to reduce HIV risk among PWID may also reduce their risk of facilitating injecting initiation. This addresses a gap in HIV prevention research and represents a novel and potentially groundbreaking research area with major implications for HIV prevention.